alm

BTW , if the resistance of the percsision resistor changes via the tem'p , canno't we compenset that via thermistor/thermo couple ? (IE , define that every 0.1C = 0.02ppm to add or something like this ..)

This trick is used by TCXOs (temperature compensated crystal oscillators). Sometimes components with an opposite tempco are used in series to decrease the net tempco. The trouble with sensing the shunt resistor temperature is that you need to measure the resistor temperature accurately. Since the resistor is dissipating power, its core will always be hotter than its case. You could model this with the thermal resistance, but it's probably more trouble than it's worth, a more expensive resistor with a lower tempco is probably cheaper. Putting the shunt resistor in an oil bath appears to be more common, but is not very practical in a handheld meter.

He claims the green backlight is incandescent? Whaaat? That makes no sense. The 87-III is from when, the late 90s/early 2000s? Green LEDs were around then. In fact much earlier. It would make zero sense for Fluke to use incandescent bulbs. Also, in his photo, you can see that the backlight consists of four light sources. If they used light bulbs, they wouldn't have used four of them. On top of that, an incandescent backlight wouldn't be green, unless they painted the bulbs green.

My meter is going to arrive in a few hours. If it has the green backlight, I'll do some current measurements, take it apart and check it out.

So the 87-III just arrived. Too bad, it's a white backlight. By the way, it does NOT turn off automatically, just like on the 87V. It only has one brightness level, though (about as bright as the high level of the 87V). I also did the resistance check aginst the mA jack: 1.0012k.

This is an incandescent backlight module from an original 87 (circa 1990). Sorry for the bad focus, I'm in a bit of a hurry here. Not sure where the green color comes from, it's sealed up and I don't wish to break it.

It draws 60mA at 8.5V.

Edit:To be honest, the only reason I called it incandescent is because I had it in a container labeled "old 87 incandescent backlight module". I just did a quick test and it draws practically no current until a knee when the terminal voltage reaches 3.0V. Which sounds more like there's actually some arrangement of LEDs on the other side. In any case, it is a power hog.

Obviously it's 2x2 green LEDs in series at 30 mA each. You could probably replace those 130 ohm resistors with 1k ones and it would still be bright enough. Do you know if the models with the green backlight have a backlight timer?

One downside I found is that the 87-III doesn't sound the beeper in diode test mode. Bummer.

The different bargraph types of the 87-III vs the 87-V are also curious ...

Alright, I recently got a Fluke 88 (original series, with green backlight) and I wanted to get to the bottom of this. I measured the battery drain. Here are my findings:

Fluke 88 (green BL)

Fluke 87 III (white BL)

Fluke 87 V (white BL)

off

9.6µA

8.6µA

0.1µA

VDC

760µA

920µA

1.15mA

VDC, BL on

22mA

26mA

9.18mA (Level 1), 21.2mA (Level 2)

So much for "battery destroying". Another myth busted. Plus, the green actually looks cooler.

Edit: added data for 87 V.

No idea if the larger off-state drain of the older meters is due to aging (carbon from the switch causing leakage current) or due to the circuit design. IMHO it must be the latter, because all 3 meters measure within 2 counts of each other (quick check with a 9V battery: 87 V says 9.033, 87 III says 9.031, 88 says 9.032) , I think internal current leaks would have an effect on the calibration.

Edit 2: ah, found it. The older 80 series have a signal called PDS (power down signal) that is connected to VBT+ and in the off state is pulled down to VBT- through a 1M resistor. So all the older 80 series will have an off state drain of around 9 microamps. It's all good.

Obviously it's 2x2 green LEDs in series at 30 mA each. You could probably replace those 130 ohm resistors with 1k ones and it would still be bright enough. Do you know if the models with the green backlight have a backlight timer?

One downside I found is that the 87-III doesn't sound the beeper in diode test mode. Bummer.

The different bargraph types of the 87-III vs the 87-V are also curious ...

I didn't know about this before I bought it either...so I bought another 87V and eventually I'll be getting around to listing the 87 III, not just for that but another knock(don't like the holster, capacitance, temp) it's harder than I thought to adjust to the limited feature set.

In some light I do like the green light more but most of the time I prefer the white. At least it isn't orange like Agilent/Keysight, that is soo weird to me.

The 87 III does have some features that the 87 V is missing. The autoranging is much faster. But the main thing is, the 87 III's bar graph (actually an analog pointer) has 6 times the resolution of the 87 V's bar graph (1000 counts full scale vs. 6000 counts full scale). I'll keep mine for that reason alone.

In the original 80 series, the 83, 87 and 88 all had different LCDs. The 83 had a regular bar graph that is just a full scale reading of the current range. The 87 had the high resolution analog pointer (4000 counts, wrapping four times for each range). The 88 had yet another type, a 2000 count analog pointer, wrapping twice per range. So the high res pointer was actually a big feature that distinguished the 87 from the cheaper 83.

Probably to save money in production, on the 80 V series, the 83 V, the 87 V and the 88 V all use the same LCD. So you only get the regular 6000 count bar graph.